Method for determining timing differences between radio transmitters and a radio network incorporating the same

ABSTRACT

There are proposed a method for determining timing differences between a plurality of radio transmitters (BTS) and a radio network incorporating the method. Timing information of signals received by measuring means from said plurality of radio transmitters (BTS) is determined. The timing differences between said plurality of radio transmitters (BTS) are determined on the basis of said timing information determined by said measuring means and the respective distance differences between said plurality of radio transmitters (BTS) and said measuring means. This method can be advantageously applied to locating mobile stations in a mobile radio network.

FIELD OF THE INVENTION

The present invention relates to a method for determining timingdifferences between radio transmitters and a radio network incorporatingthe same. More specifically, the present invention relates to thedetermination of timing differences between base transceiver stations ina mobile radio network. This method of determining the real timedifferences is based on the measurement of the observed timedifferences.

BACKGROUND OF THE INVENTION

In mobile radio networks such as the GSM system used throughout Europe,it is necessary that the position of each mobile station is known to thebase stations with a sufficient accuracy.

There are already some methods for determining the locations of mobilestations in a mobile radio network. For many of the location methodssuch as those based on Time Difference on Arrival (TDOA) or DirectionFinding (DF), it is necessary that the base transceiver stations operatesynchronously with respect to each other. However, in the GSM mobileradio network as it is currently used in Europe, the base transceiverstations operate asynchronously with respect to each other, or thesynchronisation is not accurate enough for location purposes. For thisreason, it would be desirable to know the timing differences of thesignals transmitted by the base transceiver stations.

However, no method is available at present which would be able todetermine these timing differences. As a result, it is not possible tolocate a particular mobile station with a high accuracy.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodfor determining timing differences between a plurality of radiotransmitters in a radio network. It is another object of the presentinvention to provide a method for determining timing differences betweena plurality of base transceiver stations in a mobile radio network whichwould allow to increase the accuracy of locating the mobile stations. Itis a further object of the present invention to provide a radio networkin which such methods can be carried out.

According to the present invention, this object is accomplished by amethod for determining timing differences between a plurality of radiotransmitters in a radio network, comprising the steps of determiningtiming information of signals received by measuring means from saidplurality of radio transmitters, and determining the timing differencesbetween said plurality of radio transmitters on the basis of said timinginformation determined by said measuring means and the respectivedistance differences between said plurality of radio transmitters andsaid measuring means.

According to the present invention, this object is also achieved by aradio network as defined in claim 9.

In this way, the timing differences between the plurality of radiotransmitters can be determined. Thus, if the radio transmitters are basetransceiver stations in a mobile radio network, it becomes possible todetermine the location of any mobile station moving at some point in themobile radio network with a much higher accuracy than has been possiblein the prior art. This requires in addition that those mobile stationswhose positions are to be calculated determine timing information ofsignals received from the plurality of base transceiver stations.Furthermore, since the knowledge of the timing differences allows tooperate the base transceiver stations in a pseudo-synchronous manner,the way of effecting handovers of the mobile stations can be improved.

Advantageous embodiments of the present invention are defined in thesubclaims.

The preferred embodiments of the present invention will be described indetail hereinbelow by way of example with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the principle of determining the timingdifferences between two base transceiver stations in a mobile radionetwork according to the present invention.

FIG. 2 is a diagram showing an application example of the methodaccording to the present invention for locating mobile stations in amobile radio network.

FIG. 3 is a diagram showing a practical arrangement to which the methodaccording to the present invention can be applied for locating mobilestations in a GSM system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method according to the present invention aims at determining thetiming difference of signals transmitted by radio transmitters in aradio network. More specifically, the timing difference of signalstransmitted by base transceiver stations in a mobile radio network canbe determined. This timing difference corresponding to thesynchronisation difference between the base transceiver stations isreferred to as the real time difference. In theory, when the network issynchronised, two base transceiver stations should send TDMA bursts atexactly the same time, i.e. the synchronisation error between two basetransceiver stations in the network should be zero. In practice,however, the mobile radio network, like GSM, is normally notsynchronised, and the real time difference corresponds to the differenceof the transmission times of two bursts from two base transceiverstations.

The method according to the invention is based on the calculation of thedifference between the so-called observed time difference OTD and theso-called geometric time difference GTD. Namely, the observed timedifference is the sum of the real time difference and the geometric timedifference.

The different quantities used by the method according to the presentinvention will be further explained with reference to FIG. 1 whichillustrates the principle of determining the timing differences betweentwo base transceiver stations BTS1 and BTS2. As illustrated in FIG. 1,while T₁ denotes the time when a first radio signal is transmitted fromthe base transceiver station BTS1, this first radio signal is receivedby the mobile station MS at a time T₃. Accordingly, a second radiosignal is transmitted by the neighboring base transceiver station BTS2at a time T₂ and is received by the mobile station MS at a time T₄.

The observed time difference which is also referred to as the timedifference on arrival (TDOA) is the time interval as observed by themobile station between the reception of two signals from two differentbase transceiver stations. Referring to FIG. 1, the observed timedifference OTD which is continuously determined by the mobile stationitself is the difference between the two reception times T₃ and T₄:OTD=T₃−T₄. The real time difference RTD corresponds to the transmissiontime difference between the signals transmitted by the two basetransceiver stations, i.e. RTD=T₁−T₂. Since the transmission times T₁and T₂ are not known to the mobile station, the real time difference isalso not known to the mobile station.

Furthermore, the geometric time difference GTD represents the differencebetween the propagation times of the two signals transmitted by the twobase transceiver stations and received by the mobile station. Accordingto the situation illustrated in FIG. 1, GTD=T₃−T₁−(T₄−T₂). In otherwords, the geometric time difference corresponds to the respectivedifference of the distances d₁ and d₂ of these two different basetransceiver stations from the mobile station, divided by the speed ofthe radio wave propagation c, i.e. GTD=(d₁−d₂)/c. If the two basetransceiver stations are located at the same distance from the mobilestation, then GTD is zero, and the observed time difference is only dueto synchronisation differences in the network. In addition, when thebase transceiver stations are synchronised with each other (i.e. RTD=0),then OTD equals GTD, and the observed time difference is only due to thedifferent positions of the base transceiver stations and the mobilestation.

In addition, in order to determine the observed time difference betweenthe signals transmitted by the base transceiver stations and tocalculate the real time difference on the basis thereon, a dedicatedmeasurement equipment is used with which some of the base transceiverstations acting as fixed measurement stations are equipped.

According to this embodiment of the present invention, this measurementequipment is the so-called Site Test Mobile (STM) manufactured by Nokia.As a matter of course, the present invention is by no means limited tothe use of this type of measurement equipment STM, and any other type ofsuitable measurement equipment may also be used instead. In addition,the measurement equipment does not need to be mobile, but can be locatedin a fixed position. For the sake of clarity, only a single measurementequipment STM is shown in FIG. 1 at a place located somewhere betweenthe two base transceiver stations BTS1 and BTS2.

The method according to the present invention can be advantageouslyapplied to locating mobile stations in the area covered by the mobileradio network. Referring to FIG. 1, for calculating a location estimateof the mobile station MS in the mobile radio network, it is necessary todetermine the difference between the distances d₁ and d₂ which can beobtained from the geometric time difference of the mobile station withrespect to the two base transceiver stations. As a matter of course, inorder to calculate the location of the mobile station, it is necessaryto receive at least one further signal from a third base transceiverstation which is, for the sake of clarity, not shown in FIG. 1.

More specifically, the principle underlying the application of thismethod to locating mobile stations will be described hereinbelow withreference to FIG. 2. The calculation of the location of the mobilestation according to this method is based on the fact that the possiblelocation of the mobile station MS between two neighbouring basetransceiver stations BTS is positioned on a hyperbola since thedifference between the distance d₁ of the mobile station to the firstbase transceiver station and the distance d₂ thereof to the second basetransceiver station is constant. In other words, the mobile stationconcerned is subject to a constant value of the observed time differenceof the signals received from these two base transceiver stations. Sincethe measurement result has a certain error margin, the potentiallocation area of the mobile station is in reality a band between twohyperbolae, the width of the band depending on the error margin of themeasurement result. The possible location area is illustrated by a greyarea in FIG. 2, while the dashed line represents the measured observedtime differences. In order to actually locate the mobile station, it isnecessary to receive signals from at least three base transceiverstations. In this case, the mobile station is located in theintersection of two hyperbolae obtained from two (constant) observedtime differences from these three base transceiver stations (it is evenpossible to form a third hyperbola on the basis of the signals receivedfrom three base transceiver stations). The actual location area of themobile station is illustrated by a black area at the intersection of thegrey areas in FIG. 2. If more than three base transceiver stations andthus more observed time differences are available, the possible locationarea of the mobile station can be further reduced, thus increasing theaccuracy of locating the mobile station.

In order to actually locate a particular mobile station on the basis ofthe geometric time difference, the observed time difference OTD and thereal time difference RTD have to be known. Furthermore, in order todetermine the real time difference, the positions of the different basetransceiver stations and that of the measurement equipment STM have tobe known. The positions of the base transceiver stations arepredetermined and defined by co-ordinates in an x-y-plane. Since thelocation of the measurement equipment is also known, the geometric timedifferences GTD with respect to the measurement equipment can becalculated. Thus, since the observed time difference OTD between thesignals received by the measurement equipment is determined by themeasurement equipment, it is possible to determine the real timedifference RTD according to the method described above. This real timedifference is calculated by determining the difference between thegeometric time difference GTD corresponding to the distances of themeasurement equipment to the base transceiver stations, and the observedtime difference OTD.

Furthermore, since the real time difference is independent from theposition at which it is determined, the calculated value corresponds tothe real time difference as observed by the mobile station. Thus, afterdetermination of the observed time difference OTD by the mobile station,the derived value is transmitted to some central unit. Thereafter, thegeometric time difference can be calculated from which the distancedifferences between the mobile station and the base transceiver stationsare determined. In this way, one hyperbola can be drawn from the signalsreceived from two base transceiver stations, since one geometric timedifference can be calculated. If signals are received from three basetransceiver stations, then three hyperbolae can be drawn from twogeometric time differences, thereby allowing the possible location ofthe mobile station to be determined.

FIG. 3 is a diagram showing a practical arrangement to which the methodaccording to the present invention can be applied for determining thelocation of a mobile station in a GSM mobile radio network.

As shown by a dotted line in FIG. 3, the RTD results calculated by themeasurement equipment STM are transmitted via a base station controllerBSC, a mobile switching center MSC, a short message service-servicecenter SMS-SC to a location service center LSC. Alternatively, themeasurement equipment may only report the determined OTD results to thelocation service center which in turn calculates the real timedifference. The location service center can be either provided as aseparate entity in the mobile radio network as shown in FIG. 3, or itcan be provided in a distributed manner over the whole mobile radionetwork, e.g. in any of the base transceiver stations. In addition, asshown by a dashed line in FIG. 3, the TDOA information (or OTD results)can be relayed from the mobile station MS usually via the serving basetransceiver station BTS, the base station controller BSC, the mobileswitching center MSC, the short message service-service center SMS-SC tothe location service center LSC.

In general, the location service center then calculates the differencebetween the OTD results transmitted from the mobile station and the RTDresults transmitted from the measurement equipment STM so as tocalculate a location estimate of the mobile station MS. In other words,the location service center calculates the location of the mobilestation on the basis of the geometric time difference which is thedifference between the observed time difference and the real timedifference. The location service center then outputs the determinedposition co-ordinates of the mobile station to a number of applications(e.g. applications 1 and 2 shown in FIG. 3) which have issued a locationrequest to the location service center in advance concerning aparticular mobile station. Such a location request initiates thetransmission of an OTD request from the location service center to themobile station.

Since it is possible that the timing differences of the base transceiverstations vary from one measurement to the next, the OTD resultscalculated by the measurement equipment are periodically reported to thelocation service center.

As illustrated in FIG. 3, it is not necessary that every basetransceiver station BTS is provided with its own measurement equipmentSTM. For instance, in urban areas where many base transceiver stationsare located close to each other, it may be sufficient to provide onlye.g. every sixth base transceiver station with a measurement equipment.

Furthermore, according to another preferred embodiment of the presentinvention, the mobile station MS can determine its own position itself.For this purpose, the real time differences RTD calculated by either themeasurement equipment or the location service center are transmitted tothe mobile station. The mobile station determines the timing informationof the signals received from several base transceiver stations, i.e. theobserved time differences, and calculates the geometric time differencein the same manner as described above. On the basis of the geometrictime difference, the mobile station is able to determine its position onits own.

More specifically, the calculated real time differences RTD or theobserved time differences OTD can be transmitted from the measurementequipment STM to the location service center LSC using any of thefollowing four channels:

the short message service (SMS),

undefined supplementary service data (USSD),

a measurement report, a BSSMAP message and a message between a mobileswitching center MSC and the location service center LSC, or

a totally new message.

The short message service is a special GSM service for delivering shorttext messages either from one mobile station to another or from acentral base station to all mobile stations in a particular geographicalregion. In practice, the short message service is implemented as aconnectionless protocol, and the messages are transmitted on one of thesignalling channels (either the Slow Associated Control Channel (SACCH)or the Stand Alone Dedicated Control Channel (SDCCH)).

Furthermore, the undefined supplementary service data (USSD) are datathe content of which is not defined by any specification. These data canbe used by an operator to implement new operator specific services.

In addition, base station system management application part (BSSMAP)messages are used in the communication between the mobile switchingcenter MSC and the base station system BSS. They support all proceduresbetween the mobile switching center and the base station system whichrequire information processing related to single calls and resourcemanagement.

According to the embodiment shown in FIG. 3, for transmission of OTDrequests and results the short message service SMS is used between thebase transceiver stations BTS and the mobile station MS. Furthermore,according to this embodiment, the SMS channel is also used fortransmission of the RTD results in the uplink direction from themeasurement equipment STM to the location service center LSC.

The use of the SACCH channel for transmission of measurement informationhas the following advantages. Since a SACCH burst is transmitted atintervals of 480 ms in the GSM system, the measurement information istransmitted sufficiently often. Further, the measurement information caneven be transmitted during an ongoing call. Moreover, since an alreadyexisting channel is used for transmission of additional information,neither the load on the radio link is increased nor the quality of thetransmitted voice signals is impaired.

The measurement process in the measurement equipment STM can betriggered and controlled by sending a special measurement command signalas RTD request or command using e.g. the short message service or theundefined supplementary service data to be sent to the measurementequipment. The transmission of the command signal using the shortmessage service is again represented by the dotted line in FIG. 3. Thiscommand signal may contain the following information:

the number of measurement sets of one SMS message,

the measurement frequency (e.g. once every 30 seconds),

the measurement duration (e.g. the next ten minutes, five SMS messagesor until a stop command is received), and

the identification of those base transceiver stations BTS beingmeasured.

Hence, according to the present invention, since the real timedifference between the plurality of base transceiver stations can beaccurately determined, it becomes possible to precisely determine thelocations of mobile stations being positioned somewhere in the mobileradio network. In detail, experiments have shown that it is possible toreduce the mean error of mobile station location to approximately 300 m.Furthermore, the position information of a mobile station can beobtained quickly and at short intervals because the position informationbecomes available in real time.

Furthermore, according to the above description, the measurementequipment is located in the base transceiver station. This has thefollowing advantages. Since the antenna for receiving the signals islocated at the antenna of the base transceiver station, it has in manycases a line of sight connection to other base transceiver stations. Asa result, the attenuation between signals received from neighbouringbase transceiver stations is low. Furthermore, since only some softwaremodifications in the mobile station are needed, only little additionaleffort is required. Thus, the present invention can be realized atalmost no extra cost. In addition, the measurement equipment may ofcourse also be used for other purposes. However, it is also notnecessary that the measurement equipment is located in the basetransceiver station, and the measurement equipment may of course belocated in some other suitable place.

In the above description, the application of the method to basetransceiver stations in a mobile radio network has been described as anexample. However, the method according to the present invention can as amatter of course be applied to determine the timing differences betweenseveral radio transmitters in any type of radio network.

Furthermore, although the above description referred to the Pan-Europeandigital cellular radio network GSM as an example, the present inventioncan be applied with some modifications to any other digital system. Forexample, the present invention is also applicable to the DCS1800/1900system or the JDC system used in Japan.

It should be understood that the above description has been made onlywith reference to the preferred embodiments of the present invention.Therefore, the present invention is not limited to the above describedpreferred embodiments, but is also intended to cover any variations andmodifications to be made by a person skilled in the-art within thespirit and scope of the present invention as defined in the appendedclaims.

What is claimed is:
 1. A method for determining timing differencesbetween a plurality of radio transmitters, comprising the steps of:determining timing information of signals received by measuring meansfrom said plurality of radio transmitters; and determining the timingdifferences between said plurality of radio transmitters on the basis ofsaid timing information determined by said measuring means, providingsaid measuring means at a known location; performing said determiningstep of determining timing differences by using respective distancedifferences between said plurality of radio transmitters and saidmeasuring means; and controlling said determination of said timingdifferences by transmitting a command signal from a location servicecenter.
 2. A method according to claim 1, wherein said radiotransmitters are base transceiver stations in a mobile radio network. 3.A method according to claim 2, further comprising the steps of:determining timing information of signals received by said mobilestation from said plurality of base transceiver stations; anddetermining the location of said mobile station on the basis of thetiming information determined by said mobile station and said timingdifferences between said plurality of base transceiver stations.
 4. Amethod according to claim 3, wherein said timing information istransmitted using the short message service, undefined supplementaryservice data or a BSSMAP message.
 5. A method according to claim 3,wherein said timing differences are transmitted using the short messageservice, undefined supplementary service data or a BSSMAP message.
 6. Amethod according to claim 3, wherein said command signal is transmittedusing the short message service or undefined supplementary service data,and wherein said command signal contains some of the followinginformation: the number of measurement sets of one message, themeasurement frequency, the measurement duration, and the identificationof those base transceiver stations being measured.
 7. A method accordingto claim 1, wherein one or several of said plurality of radiotransmitters are provided with said measuring means.
 8. A methodaccording to claim 1, wherein said measuring means is/are arranged at adistance from any of said plurality of radio transmitters.
 9. A radionetwork comprising: a plurality of radio transmitters; and measuringmeans for determining timing information of signals received from saidplurality of radio transmitters, wherein said measuring means isarranged to determine the timing differences between said plurality ofradio transmitters on the basis of said timing information determined bysaid measuring means, said measuring means is provided at a knownlocation; and said measuring means is arranged to perform saiddetermination of said timing difference on the basis of respectivedistance difference between said plurality of radio transmitters andsaid measuring means, wherein said determination of said timingdifference is controlled on the basis of a command signal received froma location service center.
 10. A radio network according to claim 9,wherein one or several of said plurality of radio transmitters areprovided with said measuring means.
 11. A radio network according toclaim 9, wherein said measuring means is/are arranged at a distance fromsaid plurality of radio transmitters.
 12. A radio network according toclaim 9, wherein said radio network is a mobile radio network and saidradio transmitters are base transceiver stations.
 13. A radio networkaccording to claim 12, wherein a mobile station is adapted to determinetiming information of signals transmitted from said plurality of basetransceiver stations; and the location of said mobile station isdetermined on the basis of said timing information determined by saidmobile station and said timing differences between said plurality ofbase transceiver stations.
 14. A radio network according to claim 13,wherein said location service center is adapted to determine thelocation of said mobile station (MS).
 15. A radio network according toclaim 14, wherein said location service center is spatially distributedover said mobile radio network.
 16. A radio network according to claim14, wherein said measuring means is adapted to transmit said timinginformation to said location service center or said mobile station isadapted to transmit said timing information to said location servicecenter, wherein said transmission of said timing information is effectedusing the short message service, undefined supplementary service data ora BSSMAP message.
 17. A radio network according to claim 14, whereinsaid timing differences are transmitted from said measuring means tosaid location service center using the short message service, undefinedsupplementary service data or a BSSMAP message.
 18. A radio networkaccording to claim 13, wherein said command signal is transmitted byusing the short message service or undefined supplementary service data,and wherein said command signal contains some of the followinginformation: the number of measurement sets of one message, themeasurement frequency, the measurement duration, and the identificationof those base transceiver stations being measured.
 19. A mobile stationadapted to be located in a mobile radio network according to claim 12,wherein said mobile station comprises determination means adapted todetermine the position of said mobile station on the basis of timinginformation of signals received from a plurality of base transceiverstations and timing differences between said plurality of basetransceiver stations, transmitted to said mobile station, wherein thetiming differences are determined by measuring means of the network onthe basis of respective distance differences between said plurality ofbase transceiver stations and said measuring means, the determining ofsaid timing differences being controlled on the basis of a commandsignal received from a location service center.